Engines may be configured with boosting devices, such as turbochargers or superchargers, to increase airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. While a turbocharger includes a compressor that is mechanically driven by an exhaust turbine, an electric supercharger includes a compressor that is electrically driven by a motor. In some engine systems, one or more intake charging devices may be staged in series or parallel in what may be referred to as a compound boosting configuration. For example, a fast, auxiliary boosting device (e.g., the electric supercharger) may be utilized to increase the transient performance of a slower, primary boosting device (e.g., the turbocharger). In such a configuration, the turbocharger may be upsized to increase peak power and torque performance of the engine, which enables more aggressively downsized engines.
Various approaches may be used to provide boost control in a compound boosting system. One example approach for compound boosting system control using pressure ratios is shown by Petrovic et al. in EP 1,927,739 A1. The pressure ratio may represent the boosting capability of a boosting device of the compound boosting system. In the approach of Petrovic, a method for coordinating two turbochargers based on desired partial pressure ratios is disclosed. Specifically, the desired partial pressure ratios for each turbocharger are determined based on calibrated look-up tables using engine speed and engine torque as inputs. The desired partial pressure ratios are then achieved through at least one of adjusting a turbocharger wastegate opening, adjusting turbine vane geometry (e.g., if a variable geometry turbine is included), and adjusting openings of turbine and/or compressor bypasses.
However, the inventors herein have recognized potential issues with such systems. As one example, it is a static approach that uses predefined calibrations to determine the desired partial pressure ratios, which may be independent of one another (e.g., the desired partial pressure ratio of one compression device does not influence the desired partial pressure ratio of the other compression device). If the approach of Petrovic were applied to a compound boosting system including an electric supercharger staged alongside a turbocharger, the approach may cause the supercharger to be run for a longer than required duration, resulting in a drop in fuel economy. In addition, there may be conditions where the target boost pressure is transiently overshot, such as when an intake throttle opening is transiently increased. This can result in a waste-gate being opened prematurely. As a result, the turbocharger turbine may start spinning down and subsequent attainment of the target boost pressure may be delayed.
The inventors herein have recognized that a total pressure ratio across a compound boosting system can be achieved as a product of the pressure ratios across each compression device arranged in series. The total pressure ratio can be determined as a function of the desired boost pressure, which is a function of the torque demand. The multiple compression devices of the compound boosting system may include at least a slower acting (or lower frequency) compression device (herein also referred to as a primary device) and a faster acting (or higher frequency) compression device (herein also referred to as an auxiliary device). By dynamically allocating a portion of the total pressure ratio (or desired boost pressure) to the auxiliary compression device based on the capability of the primary compression device, a desired boost pressure can be attained more efficiently.
In one example, the issues described above may be addressed by a method comprising: responsive to driver torque demand, generating an overall pressure ratio command for a first, slower, compression device of an engine intake; and adjusting a pressure ratio command for a second, faster, compression device in the engine intake responsive to a boost pressure shortfall required for the driver torque demand. In this way, a target boost pressure can be reached faster and more efficiently in a compound boosting system.
As one example, a compound boosting system may include an upstream, faster-acting, auxiliary compressor (e.g., an electric supercharger compressor) and a downstream, slower-acting, primary compressor (e.g., a turbocharger compressor). Responsive to an operator torque demand, an engine controller may dynamically allocate pressure ratios to each compressor to meet the demand. In particular, an overall pressure ratio command may be generated for the turbocharger. The overall pressure ratio command may include corresponding adjustments to an opening of an exhaust waste-gate valve coupled in a waste-gate across the turbocharger turbine. For example, as the torque demand increases, the waste-gate opening may be decreased to direct more exhaust flow through the turbine, spinning up the turbine to spin up the turbocharger compressor. However, due to the slower response time of the turbocharger, there may be a shortfall in the boost pressure delivered (by the turbocharger) to meet the torque demand. The controller may then generate a pressure ratio command for the electric supercharger that is based on the boost pressure shortfall. Further, as the turbine spins up and the boost pressure shortfall at the turbocharger decreases, the pressure ratio commanded to the turbocharger relative to the supercharger may be dynamically updated.
In this way, by setting the pressure ratio of a higher frequency auxiliary compressor based on the boost pressure shortfall of a lower frequency primary compressor, an overall pressure ratio target may be achieved more efficiently. By coordinating an auxiliary supercharger operation with a primary turbocharger operation, premature waste-gate opening is reduced. The technical effect of using a dynamic approach to allocate the pressure ratios is that the pressure ratio of the supercharger can be continually updated as the boosting capability of the turbocharger compressor changes, reducing the duration of operation of the electrical supercharger without compromising boost output. Further, boost pressure overshoot and undershoot may be avoided. In this way, unnecessary activation of the supercharger may be minimized, and electric power (which may be limited) may be conserved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.